Prepare a 9x9-inch baking dish by lining it with parchment paper to aid in fudge removal.

In a large saucepan over low heat, combine white chocolate chips and half of the sweetened condensed milk. Stir continuously for about 5 minutes until melted and smooth.

Remove from heat, stir in peppermint extract, and pour half of the mixture into the prepared dish, smoothing it evenly.

In a separate bowl, melt semi-sweet chocolate chips with the remaining sweetened condensed milk over low heat for about 5 minutes, then pour over the white chocolate layer.

Gently use a skewer or knife to swirl the layers together to create a marbled effect.

Sprinkle crushed peppermint candies on top before it sets, optionally adding red food coloring and swirling again.

Cover with plastic wrap and chill in the refrigerator for at least 2-3 hours until firm.

Lift the fudge from the dish using the parchment paper, cut into squares, and serve or package as gifts.